The landing gear compartment is used to housing landing gear on any type of aircraft, in particular when the landing gear is placed under the fuselage of the aircraft.
Usually, this landing gear compartment is closed by several doors. For example, and in reference to FIG. 1, the compartment 20 for the front landing gear 30 is in particular closed by:                the front doors 40, which open during the descent of the gear 30, in particular to allow the leg 31 thereof to pass, and close after the exit of the gear 30 so as to preserve the aerodynamism of the fuselage 11 of the aircraft 10, and        the back doors 50, which open during the descent of the gear 30 and remain open as long as the gear 30 is out.        
A landing gear door is usually formed by a single-piece rigid panel. This panel is generally hinged on the aircraft structure around a pivoting axis offset towards the inside of the gear compartment in relation to said panel. More precisely, the panel is mounted secured to the gear, such that the exit and return of the gear cause the opening and closing of the panel, respectively.
The pivoting of the door is usually obtained via a direct mechanical connection between the door and the gear.
The direct connection consists of a single connecting rod mounted, on one hand, on the landing gear, for example on the leg, the beam or the strut, and on the other hand on the door. Thus, when the gear comes out, it pivots around an axis causing the movement of the connecting rod, which makes the door pivot into its opening position. In parallel, the return of the gear into the compartment causes the door to close.
The use of a single connection does, however, have the drawback of presenting a limited freedom of positioning of the connecting rod, implying significant design constraints.
Indeed, the connecting rod must be mounted on the gear on one hand and on the door on the other.
Among these constraints, we sometimes see the obligation to arrange the connecting rod inclined relative to the door, due in particular to the reduced space in the landing gear bay and the kinematics of the landing gear and the door.
One consequence is the low intensity of the restoring moment exerted by the connecting rod on the door, when the door is in the closed position. The door indeed undergoes very significant aerodynamic stresses that the connecting rod cannot completely balance.
Thus, to pick up the stresses undergone by the door, the fittings attaching the door to the structure of the compartment, for example goosenecks, are reinforced and then have large dimensions. The problems of mass and bulk then arise.
Moreover, before the landing gear comes out and when the front doors are open, the air rushes into the landing gear compartment and also exerts a very significant stress on the back doors.
The back doors vibrate greatly, which causes, aside from noise problems, premature fatigue of the doors and the corresponding connecting rods.